This invention relates to a process for estimating a property of use, for example the activity of a catalyst or the ability to hold a radio-element in a solid mineral matrix, of a material MAB whose active element is AB. The invention also relates to a process for determining the chemical affinity of an element or set of elements B for a matrix A, for example the affinity of a material for oxygen or sulfur-containing compounds or halogenated compounds. This more or less significant affinity makes it possible, for example, to identify the resistance of this material to corrosion by sulfur-containing compounds or halogenated compounds or by oxidation. Many other applications of these processes can be considered, some of which are explained below. More generally, the processes according to the invention therefore make it possible to select or to design new materials whose use produces the formation or the modification of at least one chemical bond that is characterized by a descriptor DAB or makes it necessary to prevent the formation of said bond.
In the present prior art, the selection or the design of materials for a determined application is considered only on an experimental basis according to the trial-and-error method. This practice is obviously long and expensive, and any process that allows a significant reduction of this search phase would offer a technical and economic advantage.
Numerous properties of use of the materials are to a large extent directly determined by the forces of chemical cohesion that are inherent to their composition: this is the case, for example, of mechanical properties (modulus of elasticity, resistance to rupture, hardness . . . ) of metals and their alloys, ceramics, construction materials, or else the case of solubilities of host elements, used for, for example, the capture of radioactive elements in mineral structures for storage purposes. These chemical cohesion forces will also determine all of the surface properties of the materials, of which one skilled in the art knows the technological importance: friction coefficient, resistance to wear, corrosion behavior, resistance to oxidation, adhesiveness, wettability, catalytic activity . . . .
The chemical cohesion forces also govern the local atomic structure of a material and thereby its electronic structure and all the physical properties (electronic, optical, magnetic . . . ) that are derived therefrom. The search for new high-temperature superconductive phases of electric current or else the search for new solid electrolytes with improved ionic conductivity for the production of more efficient fuel cells thus amount to searching for chemical compounds that have a special local organization (see, for example, J. B. Goodenough, Nature, Vol. 404, 20/04/2000, pp. 821-822, and cited references).
The practician in the search for new materials for a given application relies as much as possible today on the knowledge and the methods developed by the scientific discipline that is the chemistry of the solid: the latter quantifies the relative stabilities of the structures under given temperature and pressure conditions on the basis of the standard concept of formation enthalpy.
The standard formation enthalpies of a very large number of compounds have been measured experimentally and tabulated; they make it possible, for example, to construct so-called useful xe2x80x9cphasexe2x80x9d diagrams for the purpose of locating the areas of experimental conditions inside of which the structures of interest remain stable. These data and diagrams therefore have a limited value for the invention of new stable phases in an area of use that is specified at the very most so that one skilled in the art can therefore extrapolate by chemical analogy and intuition starting from known structure and composition phases.
For the purpose of guiding his action logically, the chemist that practices the synthesis of organic or inorganic compounds worked out early on the concept of chemical affinity and then, when the atomic structure of the material had been well established, the concept of interatomic force was developed. Modern theoretical chemistry has as its central object the elaboration of a quantitative and predictive theory of the chemical bond within atomic, molecular or crystalline structures.
Quantum physics provided the basis of a mathematical theory whose extreme precision is verified the better and in a broader range in proportion as the increase of power of electronic computers allows the digital resolution of constituent equations for increasingly more complex chemical compositions. These so-called xe2x80x9cab initioxe2x80x9d calculation techniques, since they were unencumbered by prior knowledge of empirical data, were developed in less than two decades to the extent that it became conceivable to use them to predict the stability, the geometry and the physical and chemical properties of a chemical structure of given composition, prior to any laboratory attempt at synthesis.
This xe2x80x9cdesign of computer-assisted materialxe2x80x9d is a very active methodological area of research but of which a very limited number of practical successes is known. These successes are confined to special cases, for example the development of a hydrocarbon reforming catalyst with a metallic nickel-based vapor and with increased stability by selective deposition of gold atoms on the surface (F. Besenbacher et al. Science, Vol. 279, 1913-1915, Mar. 20, 1998) or else the demonstration of a cathode composition that significantly improves the voltage and reduces the weight and the cost of a lithium battery (G. Ceder et al. Nature, Vol. 392, 694-696, Apr. 16, 1998). These recent cases of success rather exemplify an approach of verification by the calculation of a design of intuitive origin, confirmed a posteriori by experimental measurement.
The economic advantage of such paths is not clearly demonstrated currently, but anyone skilled in the art will impart to them a fundamental superiority in exploratory experimentation by trial and error, whose implementation will depend on the speed and the cost of the calculations to be used.
In this connection, the very fast growth over time of the calculation power at consistent cost, because of the advances in the technologies for integrating electronic circuits, suggests decisive breakthroughs in the near future. The process according to the invention unexpectedly anticipates in this direction, as a process for fast ab initio calculation of quantitative descriptors of the chemical bond in crystalline solids, that makes it possible to classify the latter by order of efficiency for a large number of applications of primary technological importance.
A strategy for exploratory searching for new materials that it is possible to consider as diametrically opposed to the xe2x80x9cdesign of computer-aided materialsxe2x80x9d defined above consists of the xe2x80x9ccombinatorial chemistryxe2x80x9d that appeared several years ago (see, for example, U.S. Pat. Nos. 5,959,297 and 5,985,356) and that makes sense only when combined with so-called xe2x80x9chigh-flow experimentationxe2x80x9d techniques. In this case, the idea is to explore systematically by experiment a predefined space of compositions and synthesis conditions. The materials that result from these systematic combinations are prepared in very small quantities, just enough for tests that make possible a sorting according to the desired property or properties. The combinations that pass the tests make it possible to redefine a more restricted exploration space within which can be reiterated the combinatorial synthesis procedure and test for the purpose of refining the identification of combinations consistent with the initial target. The combination or combinations that are discovered are then synthesized in larger quantities to measure their properties of use with precision.
The xe2x80x9ccombinatorial chemistryxe2x80x9d approach was recently the subject of considerable financial investments having led to significant technological developments. In this context, the computer technologies facilitate the management and the tracing of the properties of a large number of samples that are synthesized and tested very quickly. They also make it possible to guide the generally robotized process of synthesis and testing at high speed. The targets of the xe2x80x9ccombinatorial chemistryxe2x80x9d to date have been, for example, new molecular medications, new photophore materials (U.S. Pat. No. 6,013,199), new polymerization catalysts (U.S. Pat. Nos. 6,034,240, 6,043,363), new materials with giant magnetoresistance (U.S. Pat. No. 5,776,359) or else new heterogeneous catalysts (S. M. Senkan in Nature, Vol. 394, pp. 350-353, Jul. 23, 1998).
The practicians of xe2x80x9ccombinatorial chemistryxe2x80x9d have fairly quickly recognized that the blind exploration of a field of combinations generally has an extremely low success rate, which runs the risk of not being balanced enough by the very large number of experiments. Methods for improvement that amount to guiding the exploration by elements of a priori knowledge have therefore been proposed. A sorting cycle can also be considered as an input of knowledge able to better guide the next sorting cycle. For this purpose, Baerns et al. thus demonstrated, for example, the advantage of a so-called artificial evolution procedure (Baerns et al., Confxc3xa8rence sur les approches combinatoires pour la dxc3xa9couverte de nouveaux matxc3xa9riaux (xe2x80x9cCombinatorial Approaches for New Materials Discoveryxe2x80x9d), organized by xe2x80x9cThe Knowledge Foundationxe2x80x9d (Fondation du Savoir), San Diego, Calif., USA, Jan. 23-25, 2000).
Another method consists in developing structure-property quantitative relationships (RQSP) by correlating a performance index according to the targeted property with a set of digital parameters that identify each chemical compound and are called descriptors. The descriptors are generally obtained from theoretical calculation: molecular weight, molecular volume, factors of geometric shape, moments of mean charge distribution, topological indices, see, for example, J. M. Newsam, in xe2x80x9cCatalyse combinatoire et haut dxc3xa9bit de conception et d""xc3xa9valuation de catalyseursxe2x80x9d (Combinatorial Catalysis and High Throughput Catalyst Design and Testing), publication series NATO ASI, Editor E. G. Derouane, Editeur Kluwer Academic, Dordrecht, 1999. The methods of modern linear or non-linear regression often make it possible to establish good correlations between performance index and a multivaried mathematical function of a limited set of descriptors. Such correlations make it possible to orient the combinatorial search for the chemical structures whose theoretical descriptors maximize the function that models the performance index. The method of the theoretical descriptors, however, is currently applied almost exclusively to molecular compounds, and a descriptor example is not known for crystalline materials.
The author of this invention already published works that describe a primitive descriptor design of the metal-sulfur bond energy in transition metal sulfides and its use for characterizing the catalytic activity of such sulfides (see H. Toulhoat et al. in xe2x80x9cCatalysis Todayxe2x80x9d (La Catalyse Aujourd""hui), V50, p. 629-636, 1999 and Patent FR-2,758,278). This primitive descriptor, however, is defined there as the ratio of the cohesion energy of the solid per unit cell to the number of bonds of the type considered identifiable by unit cells. This definition is different from the definition that is given for the descriptor according to this invention and is not derived from it.
This invention describes how a family of theoretical descriptors of the chemical bond between atomic pairs in any crystalline solid can be used to find new solids with a determined use. These new descriptors unexpectedly have a predictive capacity for multiple properties of technological use of crystalline solids, such as, for example, the catalytic activity or else the capability for storage of radio-elements.
This invention pertains to any form of exploratory search for new materials whose desired properties can be correlated with the descriptors whose calculating method is specified. It has a very special advantage when new techniques for high-flow-rate synthesis and sorting are implemented, in particular for the purpose of finding active materials in the form of crystalline or partially crystalline solids.
The invention relates to a process for estimating a given property of use of a material MAB whose active element is AB, starting from a descriptor. This descriptor is a calculated quantity, bound to each material, and can be correlated to the property of use of said material. This property of use is estimated with an index RAB that can be determined with the process according to the invention. Thus, when the use of the material is the catalysis, its property of use (catalytic activity) can be quantified thanks to the measurement of the speed of the catalyzed reaction or the conversion. When the property of use that is studied is the corrosion resistance of a material, this property can be quantified with, for example, the speed of oxidation of said material. Said descriptor has the dimension of an energy and is considered as representative of the chemical bond energy between an element or set of elements B and the element or set of elements A, in a material of general formula AB.
The process according to the invention is therefore a process for estimating a property of use of a material MAB whose active element is AB. It is thus possible, thanks to the process according to the invention, to determine an index RAB that constitutes an estimation of the property of use of material MAB.
Said process comprises the following stages:
a) Determination of the value of descriptors DXY for a set of materials MXY whose active element is XY and whose index RXY that measures the property of use of said material is known,
b) diagram or mathematical expression of correlation RXY=f(DXY),
c) calculation of descriptor DAB for material MAB,
d) determination of index RAB that constitutes an estimation of the property of use for material MAB by relating value DAB to correlation RXY=f(DXY), or by using the mathematical expression of said correlation.
The invention also makes it possible to determine the chemical affinity of an element or set of elements B for another element or a matrix A that consists of a set of elements, for example, the affinity of carbon for a metal for forming a carbide, or the affinity of oxygen with regard to a metal for forming an oxide.